Age-related macular degeneration (AMD) is the leading cause of irreversible vision loss in the developed world (for reviews see Zarbin, 1998, 2004; Klein et al., 2004; Ambati et al., 2003; de Jong, 2004; van Leeuwen et al., 2003) affecting approximately 15% of individuals over the age of 60. An estimated 600 million individuals are in this age demographic. The prevalence of AMD increases with age; mild, or early forms occur in nearly 30%, and advanced forms in about 7%, of the population that is 75 years and older (Klein et al., 1992; Vingerling et al., 1995a, 1995b). Clinically, AMD is characterized by a progressive loss of central vision attributable to degenerative changes that occur in the macula, a specialized region of the neural retina and underlying tissues. In the most severe, or exudative, form of the disease neovascular fronds derived from the choroidal vasculature breach Bruch's membrane and the retinal pigment epithelium (RPE) typically leading to detachment and subsequent degeneration of the retina.
AMD, a late-onset complex disorder, appears to be caused and/or modulated by a combination of genetic and environmental factors (Seddon and Chen, 2004; Tuo et al., 2004; Klein and Francis, 2003). Familial aggregation studies have estimated the genetic component to be primarily involved in as much as 25% of the disorder (Klaver et al., 1998a). According to the prevailing hypothesis, the majority of AMD cases is not a collection of multiple single-gene disorders, but instead represents a quantitative phenotype, an expression of interaction of multiple susceptibility loci. The number of loci involved, the attributable risk conferred, and the interactions between various loci remain obscure.
Linkage and candidate gene screening analyses have provided limited insight into the genetics of AMD. Reliable association of one gene with increased risk, ABCA4 (Allikmets et al., 1997) and one gene with decreased risk, ApoE4 (Klaver et al., 1998b, Souied et al., 1998) for AMD have been reported. In addition, several groups have reported results of genome-wide linkage analyses (reviewed in Tuo et al., 2004; Weeks et al., 2004). Linkage of one family with AMD phenotype to a specific chromosomal region, 1q25-q31 (ARMD1) has been documented (Klein et al., 1998). HEMICENTIN-1 has been suggested to be the causal gene (Schultz et al., 2003) although its role has not been reliably confirmed. The identification of overlapping loci on chromosome 1q in several studies (Weeks et al., 2001; Iyengar et al., 2003; Weeks et al., 2004) suggests that this locus may harbor AMD-associated gene(s).
Recent studies of drusen, the hallmark ocular lesions associated with the onset of AMD, have implicated a role for inflammation and other immune-mediated processes, in particular complement activation, in the etiology of early and late forms of AMD (Hageman et al., 1999, 2001; Mullins et al., 2000, 2001; Russell et al., 2000; Anderson et al., 2002, 2004; Johnson et al., 2000, 2001; Crabb et al., 2002; Ambati et al., 2003; Penfold et al., 2001; Espinosa-Heidman et al., 2003). These studies have revealed the terminal pathway complement components (C5, C6, C7, C8 and C9) and activation-specific complement protein fragments of the terminal pathway (C3b, iC3b, C3dg and C5b-9) as well as various complement pathway regulators and inhibitors (including Factor H, Factor I, Factor D, CD55 and CD59) within drusen, along Bruch's membrane (an extracellular layer comprised of elastin and collagen that separates the RPE and the choroid) and within RPE cells overlying drusen (Johnson et al., 2000, 2001; Mullins et al. 2000, 2001; Crabb et al., 2002). Many of these drusen-associated molecules are circulating plasma proteins previously thought to be synthesized primarily by the liver. Interestingly, many also appear to be synthesized locally by RPE and/or choroidal cells.
Activation of the complement system plays a key role in normal host defense and in the response to injury (Kinoshita, 1991). Inappropriate activation and/or control of this system, often caused by mutations in specific complement-associated genes, can contribute to autoimmune sequelae and local tissue destruction (Holers, 2003; Liszewski and Atkinson, 1991; Morgan and Walport, 1991; Shen and Meri, 2003), as has been shown in atherosclerosis (Torzewski et al., 1997; Niculescu et al., 1999), Alzheimer's disease (Akiyama et al., 2000) and glomerulonephritis (Schwertz et al., 2001).
Membranoproliferative glomerulonephritis type 2 (MPGN II) is a rare disease that is associated with uncontrolled systemic activation of the alternative pathway of the complement cascade. The disease is characterized by the deposition of abnormal electron-dense material comprised of C3 and C3c, proteins involved in the alternative pathway of complement, within the renal glomerular basement membrane, which eventually leads to renal failure. Interestingly, many patients with MPGNII develop macular drusen, RPE detachments and choroidal neovascular membranes that are clinically and compositionally indistinguishable from those that form in AMD, although they are often detected in the second decade of life (Mullins et al., 2001; O'Brien et al., 1993; Huang et al., 2003; Colville et al., 2003; Duvall-Young et al., 1989a, 1989b; Raines et al., 1989; Leys et al., 1990; McAvoy and Silvestri, 2004; Bennett et al., 1989; Orth and Ritz, 1998; Habib et al., 1975).
In most patients with MPGNII, the inability to regulate the complement cascade is mediated by an autoantibody directed against C3bBb. Other MPGN II patients, however, harbor mutations in Factor H (Ault et al., 1997; Dragon-Durey et al., 2004) a major inhibitor of the alternative complement pathway. A point mutation in Factor H (I1166R) causes MPGNII in the Yorkshire pig (Jansen et al., 1998) and Factor H deficient mice develop severe glomerulonephritis (Pickering et al., 2002). Moreover, affected individuals within some extended families with MPGNIII, a related disorder, show linkage to chromosome 1q31-32 (Neary et al., 2002) a region that overlaps a locus that has been identified in genome-wide linkage studies for AMD (see above). This particular locus contains a number of complement pathway-associated genes. One group of these genes, referred to as the regulators of complement activation (RCA) gene cluster, contains the genes that encode Factor H, five Factor H-related genes (CFHR1, CFHR2, CFHR3, CFHR4 and CFHR5), and the beta subunit of coagulation factor XIII. A second cluster of complement pathway-associated genes, including C4BPA, C4BPB, C4BPAL2, DAF (CD55) CR1, CR2, CR1L and MCP (CD46) lies immediately adjacent to the 1q25-31 locus.